Sodium vapor discharge lamp with infrared reflecting coating



Sept 3, 1968 R. GROTH 3,400,288

SODIUM VAPOR DISCHARGE LAMP WITH INFRARED REFLECTING COATING Filed Nov. 8, 1966 FIGJ FIG2

INVENTOR. ROLF GROTH 3,400,288 SGDHJM VAPQR DHSQHARGE LAMP WITH REFLECTHNG QQATKNG Rolf Gretta, Aachen, Tsermany, assignor to North American Philips (Co, inc, New York, N.Y., a corporation of Delaware Filed Nov. 8, 1966, Ser. No. 592,859 Claims priority, application Germany, Nov. 13, 19%;? N 27,625 4- Claims. (@i. 313-47) ABTEACT @IF THE A sodium vapor lamp provided with a heat-reflective light-transmissible coating of doped ln O on the inner wall of the outer envelope.

The invention relates to a sodium vapor discharge lamp having a transparent envelope which, preferably on the side facing the discharge tube, is coated with a layer which transmits visible light and reflects infrared radiation.

The luminous efiiciency of sodium vapor discharge lamps can be improved by using such selectively reflecting layers. The use of thin metal layers as well as of tin dioxide layers for this purpose is already known.

With gold layers of a thickness of approximately 150 A. it is possible, for example, to increase the luminous efiiciency of a sodium vapor discharge lamp by approximately 20% as compared with a lamp without an infrared reflecting layer. This higher luminous etficiency in lumens per watt is obtained only, however, if the electric power supplied to the lamp is reduced to approximately one fourth as compared with a sodium vapor lamp without a gold layer. As a result of this, the luminous flux decreases to approximately one third. This decrease of the luminous flux has two causes. First, due to the improved heat insulation a smaller current is sufficient to reach the optimum operating temperature of the discharge tube; consequently, however, the excitation density, that is to say, the amount of electrons which excite the Na atoms is accordingly smaller. In addition, losses of light occur as a result of absorption and reflection of the gold filter.

However, an improvement in this respect is obtained if a tin dioxide layer is used instead of a gold layer.

Although the reflecting power of tin dioxide for waves exceeding 4 un, which supply the essential part for the thermal radiation of sodium vapor discharge lumps is smaller than that of a gold layer, the permeability of the tin dioxide filter nevertheless is so much higher that both the quantity of light produced and the luminous efiiciency are larger than in a lamp with a gold layer.

As is known, the selective reflective behavior of highlydoped semiconductor layers in the infrared spectrum is obtained by the variation of the susceptibility of the crystal lattice, as a result of a high concentration of free charge carriers. If a high reflecting power in the infrared region is to be achieved, the concentration of the free charge carriers on the basis of theoretical considerations, must exceed mm. and the mobility of the said carriers must be as large as possible.

The manufacture of tin metal oxide on glass supports by thermal decomposition of suitable metal compounds has been known for a long time. Usually the so-called atomization method is used. In this method, the metal compound is atomized together with a suitable solvent by means of a nozzle and this mixture is blown against the hot glass support in the form of a fine mist, the conversion into metal oxide then occurring.

nied States Patent 0 M A drawback of these known methods is that the tin dioxide layers manufactured in this manner are difficult to reproduce as regards their optical and electrical properties. For example, the reflecting power in the infrared spectrum (/\=8 ,um.) fluctuates between 45 and For the mobility of the free-charge carriers values between 3 and 15 cm. /v0lt-sec. are found. The electrical optical properties in tin dioxide layers are so poorly reproducible because they strongly depend upon the temperature at which the oxide is formed.

In the normal spraying methods, the glass plates are cooled considerably during spraying as a result of the accompanying cold air. When the thickness of the layer increases the specific conductivity therefore becomes worse. With such non-homogeneous layers no reproducible results, that is to say poor reflection filters are to be expected of course. Therefore, in the manufacture of tin dioxide layers several short-lasting spraying operations are carried out in order that the supporting plates are not cooled considerably and can be heated again during the pauses in the spraying operations, or the atomized mixture must be heated beforehand, or both measures must be combined.

it has now been found that these drawbacks can be considerably mitigated in the manufacture of heat reflection filters constituted of indium oxide. In addition, the indium oxide layers show a higher reflecting power in the infrared spectrum and a higher permeability for sodium light than tin dioxide (S According to the invention, the coating layer substantially consists of doped indium oxide (In O it has been found, that in contrast with the manufacture of layers from tin dioxide it is not necessary for his purpose to take particular measures to obtain reproducible layers having good reflecting powers.

It has been found that the conductivity and the infrared reflecting power of indium oxide layers at temperatures of manufacture above 400 C. do not depend upon the manufacturing temperature. This is a decisive advantage of the indium oxide layers as compared with tin oxide layers in which the conductivity and the reflecting power vary considerably with the temperature of manufacture.

The invention will now be described with reference to the accompanying drawing, in which:

FIG. 1 shows reflection curves of prior art and reflective coatings according to the invention.

FIG. 2 shows a sodium vapor lamp according to the invention.

FIG. 1 shows the infrared reflecting power of two In O layers (curves 1 and 2) and of two Sn0 layers (curves 3 and 4) of the same thickness. The 111203 layers were manufactured at tempeartures of 500 C. (curve 3i) and 400 C. (curve 2) of the supporting plate. The SnO layers were likewise manufactured at 500 C. (curve 3) and 400 C. (curve t).

In manufacturing the layers, a solution of an indium compound was atomized through a nozzle and the atomized mixture blown in a cold condition against a hot glass plate. The temperature of the glass plate was chosen to be higher than 400 C. and preferably was between 400 C. and the softening temperatures of the glass. When using aqueous solutions only dull layers are usually obtained which are of no interest as filters. Entirely bright layers are obtained on the contrary if organic solvents, for example, butyl acetate and butanol, are used. Indium compounds, which may be used, are, for example, InCl and the other halides of indium.

For a high reflecting power in the infrared spectrum, it is necessary that the conductivity of the layers of this type be as high as possible. It should be at least larger than approximately 2X10 ohms cm.* If only indium compounds are used these conductivities cannot be obtained. For example, the conductivity when InCl alone was used, reached values only up to 2 10 ohmscmr The conductivity, and consequently the infrared reflecting power, can be increased considerably if particular dopings are added to the starting solution. The "best results were obtained with Sn-doping (in the form of SnCl and F-doping (in the form of a soluble fluorine compound, for example HF). conductivities were obtained, for example up to 42x10 ohms cm.*

Investigations into layers of different thicknesses have proved that with layer thicknesses above 0.5 ,ull'l. usually dullness occurs as a result of which the permeability for sodium light is reduced. Layers having a thickness of below approximately 0.5 ,um. are entirely absorption-free for sodium light and its permeability is only modified by interference effects. Therefore, the layer thickness should be chosen to be such that for sodium light a permeability maximum occurs. However, it was found that in the case of thin layers below approximately 0.20 m. the reflecting power decreased considerably. Particularly good results were obtained with layer thicknesses of approximately 0.31 ,um. and approximately 0.46 ,um. respectively. In both layer thicknesses a permeability of 91% is obtained for sodium light (A approximately 0.59 pm.) (permeability of the glass plate without layer approximately 9192% The reflecting power in the infrared spectnlm of the layer of 0.31 ,um. was substantially of the same value as that of the 0.46 ,um. layer. Both layers reached a reflecting power of approximately 90% and 10 m. A tin dioxide layer of 0.32 mm. thickness manufactured under optimum conditions had a permeability of 89% for sodium light and an infrared reflecting power of 80%.

The invention also will be described in greater detail with reference to the following examples and the table.

Example TABLE 1 At. psercent IL (KL) A (tl =cm. N (cm.') [1. Cmfi/volt soc.

0.03 115. 1 2.(l2 10 2. 36X10 71. 9 0.3 41. 7. 64X10 4. 2 10 71. 4 0.7 14. 8 2. 11X10 1. 97x10 66. 8 1. 5 9. 54 3. O2Xl0 3. 96x10 48. 2 1. 8 9.00 3. 48 10 4. 95 1o 43. 8 2. 3 7. 34 4. 25x10 5. 70X10 46. 6 2. 8 8. 69 3. 68 10 5. 98x10 38. 5 3. 7 9. 70 3. 23x10 5. 76X 10 35.1 5. 5 8. 3. 65X10 6. 57 (10' 34. 8

It may be seen from the table that the surface resistance R decreases with an increasing Sn-addition and reaches a minimum at approximately 2.3 atomic percent. In addition, it may be seen that the concentration of the free charge carriers N increases when the Sn-addition in creases. However, with large additions, the mobility again decreases so that maximum conductivity is reached with approximately 2.3 atomic percent Sn-addition. Additions in this range consequently are at an optimum for optimum conductivity and high infrared reflecting power. In the case of a doping with fluorine, optimum results are obtained with additions of approximately 2 atomic percent relative to indium.

The discharge tube 1 (FIG. 2) is surrounded by an evacuated glass outer envelope 4 provided with lamp sockets 2 and 3. The discharge tube includes two electrodes 5 and 6 and contains, in addition to the required quantity of sodium metal, a neon gas filling with a slight addition of argon.

Three lamps having the same geometric proportions on the inside were compared with one another. In lamp I, the outer envelope 4 was coated on the inside with a gold layer, 0.015 ,um. thick. In lamp II the outer envelope 4 was coated on the inside with a tin dioxide layer, 0.32 m. thick. This layer contained approximately 2 atomic percent of F relative to the quantity of tin as a dopant. In lamp III the outer envelope 4 was coated on the inside with an indium oxide layer according to the invention 0.31 m. thick. The layer contained 2.3 atomic percent of Sn relative to the quantity of indium as a dopant.

The measured results as shown in Table 2 were obtained:

While the invention has been described with reference to particular embodiments and applications thereof, other modifications will be obvious to those skilled in this art without departing from the spirit and scope thereof as defined in the appended claims.

What is claimed is:

1. A sodium vapor discharge lamp comprising an envelope permeable to visible radiation surrounding a discharge space in which visible light is produced by an electrical discharge through an ionizable medium containing sodium ions, an infrared radiation reflective coating on the inner wall of said envelope permeable to visible light, said layer consisting essentially of indium oxide (111 0 doped with an amount of an element taken from the group consisting of tin and fluorine at which the surface resistivity of said layer is a minimum.

2. A sodium vapor discharge lamp as claimed in claim 1 in which the coating layer contains between 1.5 and 5.5 atomic percent of tin relative to the quantity of In.

3. A sodium vapor discharge lamp as claimed in claim 1 in which the coating layer contains approximately 2.3 atomic percent of tin relative to the quantity of In.

4. -A sodium vapor discharge lamp as claimed in claim 1 in which the coating layer contains approximately 2 atomic percent of fluorine relative to the quantity of In.

References Cited UNITED STATES PATENTS 2,564,708 8/1951 Mochel 117-333 3,221,198 11/1965 Van Der Wal et al. 313-112 3,295,002 12/1966 Amans 313-108 DAVID J. GALVIN, Primary Examiner. 

